What happens to Google Docs, Sheets and Slides in my Google Drive?
Short answer: Google Docs, Sheets and Slides live in Google's own format and use little or no Drive space. The duplicate finder skips them. For portable copies, export them with Google Takeout and keep the files on your drive.
Google Docs, Sheets and Slides aren't ordinary files: they live in Google's own format and take up little or no space in your Drive. Storage Studio's duplicate finder skips them, because they carry no content fingerprint it can compare.
For a full, portable copy of your Docs, Sheets and Slides, Google's own export is the most reliable route: Google Takeout can export them as Microsoft Office or PDF files, which you can then keep on your external drive. See Google Takeout.